1. Field
The present invention concerns a method for installing a topside module on an offshore support structure that comprises at least one support leg that extends a vertical height above the water level when in use.
2. Background of the Invention
An offshore installation, such as a power plant or an oil rig, is a large structure used to house workers and/or machinery needed to drill wells in the sea floor, extract oil and/or natural gas, process the produced fluids and/or to convert renewable energy, such as wind, wave or water current energy, into a useful form, such as electricity. Moreover, the offshore installation may comprise equipment for converting electric power, e.g. from alternating current to direct current before it is conveyed to land. The offshore installation may be attached to the sea floor or an artificial island or be floating.
The deck of an offshore installation is conventionally installed on a support structure, sometimes called a “jacket structure”. The support structure may be entirely or partially immersed in water. The support structure usually includes a number of vertical or substantially vertical tubular members which constitute support legs for the deck. The deck and the support structure of an offshore installation are usually prefabricated separately on land or in a dry dock and they are then transported to a site at sea where they are subsequently assembled.
Given the large size and mass of the components of an offshore installation (where a deck alone can weigh several thousands or several tens of thousands of tons), installing the deck and other components of an offshore installation on a support structure that extends a height above the water level, has conventionally involved the use of expensive equipment, such as semi-submersible crane vessels or winching means, and is a costly, labour-intensive and complex procedure.
European patent application no. EP 654564 discloses a method for installing the deck of an off-shore platform on the support members of the support legs of a support structure. The ballastable barge, which supports the deck at a height above the support members, is brought into a position between the support legs of the support structure. The barge is then positioned so that the deck can be lowered into alignment with its corresponding support members. The barge is then ballasted to transfer the weight of the deck to the support structure. Such a method is however not suitable for installing the deck of an offshore installation on a support structure that extends a height above the water level, which height is greater than the height at which a barge can support the deck in a stable manner.